Our approach to the understanding of the mechanisms for bile acid transport involves an ongoing comparison of the transport and binding kinetics and characteristics for jejunum and ileum at each level and between levels of tissue, cellular and subcellular isolation. The kinetics and characteristics of transport for the bile acids obtained previously from in vivo and in vitro studies were tested against the same spectrum of bile acids in cellular and subcellular fraction and specific factors for transport were identified in this manner. Specifically, the rate of bile acid uptake was correlated with Na-K ATPase activity in isolated cells, and brush border member vesicles catalyzed the co-transport of Na ion and bile acid across the ileal membrane. This approach to the understanding of the intestinal mechanisms for bile acid transport was recently extended to another epithelial cell surface, the proximal convolution of the kidney. Micropuncture and brush border vesicle data indicated that the transport of bile acid in the proximal tubule, like the ileum, is the result of a secondary active, Na ion-dependent transport process involving the coupling of Na ion and bile acid flux across the brush border membrane.